Songbirds are one of the few species other than humans that learn the sounds used by their species to communicate during a sensitive period of postnatal development. Unlike humans, however, where both sexes learn and use vocal communication, in many species of songbird only males learn to sing. A striking consequence of this behavioral sex differences is that the brain regions for song control are prominent in males and diminished in size (or absent) in females. In the zebra finch, a commonly studied songbird, some vocal control regions initially develop in a monomorphic fashion in males and females. However, during the juvenile phase of song learning these regions show extensive neurodegeneration in females, while song regions in males display growth with little neuron death. What factors regulate these dramatic morphological events? Surprisingly, sex steroids appear not to act directly on the song system to influence neural fate. The proposed research focuses on the hypothesis that the development of the song system may be influenced by members of the neurotrophin family, cell-cell signaling molecules thought to regulate neuron survival and brain development in a wide variety of vertebrate species, including humans. The research will expand the knowledge base about the role of neurotrophins in regulating neuron survival, the sexual differentiation of the brain, and the development of learned vocal behavior.